Studies are being conducted on the detection of Recessive Lethal Mutations induced in the germ cells of female mice by chemical mutagens. Females were treated at various ages of foetal development and their individual male off-spring are studied. An increase in foetal mortality in backcrosses between a male and his daughters is taken as an indicator of the presence of a recessive lethal. Few studies of mutation induction in female germ cells by chemicals have been conducted, and non have utilized early germ cell stages. It is our purpose to determine if mutations can be induced in these stages, and at what frequencies they occur.